The present invention relates to heat pipes, and in particular to heat pipes formed by the extrusion of a thermally conductive material through a die resulting in an axially grooved pipe of the re-entrant type.
Conventional heat pipes operate to transfer heat from a heat source, where heat energy is produced or collected, to a heat sink, where the heat is stored or removed. The usual configuration is a closed chamber containing a working fluid which absorbs heat by evaporation and releases heat by condensation in a continuous cycle. Thus, the heat pipe may be characterized as having three sections: (1) an evaporator, located in the heat source region; (2) a condenser in the heat sink region; and (3) a transport section through which vaporized and liquid working fluid flow from the evaporator to the condenser and back.
A persistent problem in the design of heat pipes has been the provision of satisfactory means for moving the liquid working fluid from the condenser to the evaporator. Generally, such means comprise capillary flow channels in or along the walls of the heat pipe, while the central region of the pipe's cross section is reserved for vapor flow in the opposite direction.
U.S. Pat. No. 3,402,767, issued to Bohdansky, et al., discloses a heat pipe having a plurality of narrow axial grooves which by themselves serve as capillary channels to transport the condensed working fluid, avoiding the problems of a separate wicking element. However, the rectangular groove profile of Bohdansky is inefficient with respect to both the channelling of the condensed working fluid into the capillary grooves in the condenser area and in the transfer of heat through the working fluid, especially when the fluid has, as is typical, a low thermal conductivity.
The problem of optimizing the groove profiles for the evaporator, condenser, and transport sections of an axially grooved heat pipe is addressed by U.S. Pat. Nos. 3,528,494 and 3,537,514, both issued to Levendahl. In essence, Levendahl proposes a distinct profile for each section of the heat pipe. An inner wall similar to that of Peck is suggested for use in the transport section only, so as not to impair the evaporator and condenser efficiencies. Levendahl further recognizes the effect on evaporator/condenser efficiency of varying the radius of curvature of the axial groove entrances. However, the Levendahl configuration requires that the individual evaporator, condenser, and transport sections be formed separately and subsequently joined together, thus introducing considerable production costs.
In fact, production costs present a major obstacle in the design of an optimum groove profile. U.S. Pat. No. 3,566,651, issued to Tlaker, discloses a method for forming tubular parts by material displacement of the interior walls of a blank workpiece or pipe. Such deformation is accomplished by feeding the blank tube past a tapered mandrel and appropriately shaped die positioned within the tube. Another well known method for forming tubular parts is extrusion, which entails the feeding of the material from which the tube is formed past a die suspended by spider legs. The material is fed past the die in a semi-molten state, and fuses together as it passes the spider arms.
Both the Tlaker material displacement and the extrusion methods, while desirable from a low production cost standpoint, are limited with respect to the complexity of the axial groove configurations which may be formed thereby.
In U.S. Pat. No. 4,545,427 which issued Oct. 8, 1985, and is assigned to the present assignee, a re-entrant groove heat pipe is disclosed which provides an improvement of the previous axial groove configuration. In the patented device a heat pipe is provided which has a plurality of axial convergent re-entrant grooves. Capillary flow in the heat pipe is assured by the use of a capillary channel having a re-entrant groove or opening which is narrower than the central portion of the channel itself. The re-entrant groove may be readily produced by extrusion methods. However, the re-entrant groove profile must then be modified by passing a mandrel having a plurality of serrations in registry with the re-entrant grooves through the heat pipe. This modification results in a narrower entrance with tapering or convergent surfaces leading to the groove itself. The narrower entrance allows a reduction in working fluid inventory over the unmodified re-entrant groove. The convergent entrances bring about improved fluid flow into the capillary channels in the condenser section and supply appropriate surfaces in the evaporator section for the formation of thin films of working fluid to allow heat to be conducted more readily from the surface of the heat pipe to the surface of the fluid where evaporation takes place. Although the patented device operates generally satisfactorily, there is a constant demand for more thermally efficient heat pipe structures and reduced machining and modification costs after extrusion. Further, there are continuing requirements for heat pipes with reduced internal pressure as well as a reduction of pipe wall thickness. This is particularly attractive for cryogenic applications.